Fix extension definitions

Before, extensions were missing the definition of mandatory methods.
At the moment, it's not a problem because the coding guidelines are not
enforced by the code. But in the future, it will break.
Now, extensions have all mandatory method definitions.

xExtension-CustomCSS/extension.php

@@ -1,6 +1,14 @@
 <?php
 
 class CustomCSSExtension extends Minz_Extension {
+	public function install() {
+		return true;
+	}
+
+	public function uninstall() {
+		return true;
+	}
+
 	public function init() {
 		$this->registerTranslates();
 

xExtension-CustomCSS/metadata.json

@@ -2,7 +2,7 @@
 	"name": "Custom CSS",
 	"author": "Marien Fressinaud",
 	"description": "Give possibility to overwrite the CSS with a user-specific rules.",
-	"version": 0.2,
+	"version": 0.3,
 	"entrypoint": "CustomCSS",
 	"type": "user"
 }